A scanning electron microscope (hereunder, to be described as the SEM) is an apparatus for accelerating primary electrons discharged from an electron source, rectifies the primary electrons through an objective aperture, focusing the same by an objective lens to narrow down the primary electrons, scanning the sample with the primary electron beam by using a scanning deflector, detecting secondary signals generated from the sample by the irradiation of the primary electron beam, and displays the intensity of this detected signal as an observation image.
In manufacturing processes of a large scale integrated circuit, a review apparatus is usually used to analyze defects detected in an inline wafer inspection quickly and apply the result of the analysis to the removing process of the defects. This is why the review apparatus is employed widely to establish the manufacturing yield quickly and stabilize the operation of the manufacturing processes. And now that defects that might affect the semiconductor manufacturing yield adversely are getting fine and fine along with the advanced miniaturization of the manufacturing processes, there has been strongly demanded to realize an SEM review apparatus capable of reviewing the object with higher resolution than that of the conventional optical review apparatus. Basically, the configuration of the reviewing SEM is the same as that of the SEM.
In case of the review SEM, defects are classified in the following procedure. At first, an object semiconductor wafer is put on a wafer stage to fetch defect coordinates from an optical inspection apparatus. Then, the wafer is moved to the reference coordinates having the same wiring pattern as that of the defect coordinates to be focused on. The reference coordinates are observed at a low magnification. In the focusing process, the surface height of the wafer is measured with a laser measuring device to set an excitation current of the objective lens according to the measured value. After this, the wafer is moved to the defect coordinates and focused similarly to the method used for the reference coordinates to observe the defect coordinates at a low magnification. The image of the reference coordinates is then compared with the image of the defect coordinates to identify the details of the defect position, then detect a position having a difference between those two images, photograph the observed image at the defect position at a high magnification to observe the defect more in detail, and analyze the observed image automatically to classify the defect and its related foreign matters. Such a flow of operations is repeated with use of an automatic defect review (ADR) function, so that obtaining and classifying such an observed image are carried out for several thousands of defects per hour.
In order to achieve a high detection rate of defects by preventing failing of defect locating, it is required to obtain clear observed images of both reference coordinates and defect coordinates. If an observed image is dim, comparison cannot be made correctly between those images. Furthermore, because the measurement accuracy of the laser measuring device is in the order of μm, the same accuracy is also required for the focusing. The review SEM is thus designed so as to improve the accuracy more than that of the laser measuring device with respect to the focal depth that represents a distance between before and after the subject focal point to obtain clear observed images. The focal depth is in inverse proportion to the objective divergence angle, which is a half of the angle at which a primary electron beam is irradiated into the object sample.
On the other hand, the sizes of the defects to be observed are varied in the order of several tens of nm and high resolution in the order of several nm is required to observe such fine defects. Within such an objective divergence angle having a practical focal depth, the resolution is improved in proportion to the objective divergence angle.
The optical mode is optional; it can be selected properly among those having different amounts of the primary electron beam irradiated into the sample (hereunder, to be described as a probe current) and different objective divergence angles according to how the review SEM is used. As typical examples of such optical modes, there are an optical mode that takes precedence to small probe current and resolution (hereunder, to be referred to as a high resolution mode), an optical mode that takes a valance between focal depth and resolution (hereunder, to be referred to as a review mode), an optical mode that takes precedence to focal depth (hereunder, to be referred to as a long focal depth mode), etc.
In case of a small probe current, many images must be integrated to obtain an observed image with a high signal-to-noise (hereunder, to be described as the S/N) ratio, thereby the inspection throughput is lowered. In case of a high probe current, integration of less images is required, thereby the throughput is raised. In case of the high probe current, however, resolution is lowered more than the case in which the probe current is small.
Selecting an optical mode means selecting a balance between focal depth and resolution, as well as selecting a probe current. The optical mode selection is the most important parameter for the review SEM. In each optical mode, therefore, the balance between focal depth and resolution is determined severely in the design stage. And in order to determine those two performance requirements, the objective divergence angle must be set accurately. The probe current is determined by the amount of the primary electron beam that passes through the objective aperture.
In case of the review SEM described above, the objective aperture is often contaminated with foreign matters, etc. when the apparatus is started up or while the apparatus is running, thereby images are apt to be degraded. And in order to prevent this, the objective aperture is replaced once or so every year. In spite of this, the objective aperture is unavoidably variable in its diameter; the variation comes to be more than 10% among the manufacturing process and both the objective divergence angle and the probe current also come to be varied more than 10% due to the replacement of the objective aperture. Such a variation of the objective aperture diameter among the manufacturing process comes to lead to an imbalance between the focal depth and the resolution even when they are designed severely in each optical mode, resulting in generation of a performance difference between apparatuses and this makes it difficult to assure the apparatus performance.
Some general means capable of adjusting such an objective divergence angle that realizes the maximum resolution upon replacing the objective aperture with a new one have been disclosed as condenser lens adjusting methods. JP-A No. 08 (1996)-315761 discloses a technique that can adjust such an objective divergence angle by measuring the objective aperture diameter disposed between a condenser lens and an objective lens and by feeding back the measured value to the condenser lens.
In case of the configuration employed in JP-A No. 08 (1996)-315761, however, if the objective divergence angle is adjusted in accordance with an objective aperture diameter with use of the condenser lens setting device, the amount of the primary electron beam (hereunder, to be referred to as a probe current) that reaches the sample by passing through the objective aperture comes to change due to the location of the objective aperture disposed between a condenser lens and an objective lens, thereby the brightness of the image also comes to change. As a result, the desired probe current in the selected optical mode cannot satisfy the requirement of the defect inspection. This has been a problem.
Furthermore, according to JP-A No. 08 (1996)-315761, the objective divergence angle is adjusted by the condenser lens setting device to which a value is given beforehand to realize the maximum resolution. In case of the review SEM, it is required to configure the apparatus so as to realize a proper balance between focal depth and resolution and to stabilize the probe current to obtain a satisfactory detection rate of defects. Particularly, the focal depth must be set over the accuracy of the laser measuring instrument. The focal depth is the most important index to be required for the review SEM.